From this comparatively pristine vantage point, it's easy to see why naturalist Charles Darwin declared this necklace of mangroves, seagrass and submerged coral, stretching more than 150 miles off the Belize coast, "the most remarkable reef in the West Indies."

But troubles at the Western Hemisphere's largest barrier reef, like those at other "underwater rain forests" scattered across the globe, run deep.

A potent mix of coastal development, tourism, overfishing, pollution and climate change has damaged an estimated 40% of the Belize reef system, a UNESCO World Heritage Site that attracts more than a third of Belize's 850,000 annual visitors.

A recent string of "bleaching events" — where vibrantly hued coral turn a skeletal white — occurred when a spike in water temperatures that scientists associate with global warming expelled symbiotic algae living inside corals.

Worldwide, experts calculate that nearly 50% of coral reefs are under imminent or long-term threat of collapse through human pressures; 20% have been destroyed. And as a coalition of governmental and environmental groups trumpets 2008 the International Year of the Reef, anglers, snorkelers and divers from Cozumel, Mexico, to Cairns, Australia, are getting fish-eye views of an alluring but increasingly imperiled ecosystem.

In Belize, a former pirate haunt and British colony that gained independence in 1981, those dangers are particularly evident at Ambergris Caye (pronounced "key").

Once a favorite stop on the backpacker circuit and rumored subject of the Madonna hit La Isla Bonita, the narrow, 25-mile-long island has retained much of its "no shirt, no shoes, no problem" vibe. Golf carts and rusty bicycles still outnumber cars on the recently paved streets of tourist hub San Pedro, the island's only town (pop. 10,000).

Waterlogged revelers, many clutching Belize-brewed Belikin beers or concoctions made with local One Barrel rum, gather there on Wednesday nights for the "chicken drop" — a gambling exercise involving numbered squares and a defecating pollo.

But the island's dense mangroves and coastal forests, onetime shelters for jaguars, crocodiles and juvenile fish bound for the coral reef a half-mile offshore, are giving way to condos and resorts that have drawn the likes of John Grisham and the stars of the 2001, Ambergris-based reality show Temptation Island.

A casino just opened in San Pedro, and Ambergris Caye now has 107 hotels. That's double the number a few years ago, says Dorian Nuñez of the newspaper Ambergris Today, and a dozen large projects are in the works. Leonardo DiCaprio, who bought his own nearby island in 2005, reportedly plans to build an upscale eco-hotel there.

The developments' resulting sedimentation, combined with coral bleaching, hurricanes and other ills, worries locals such as Mito Paz, director of the San Pedro-based environmental group Green Reef.

"People who have never been here before go out to the reef and think it's fantastic," says Paz, 46, who grew up on the island. "But if you went to dive and snorkel sites where tour operators don't feed the fish, you'd barely see any."

That impact is visible even in such protected areas as Hol Chan Marine Reserve, a 15-minute boat ride south of San Pedro.

Established two decades ago and the most popular of Belize's 18 marine protected areas, the park draws about 54,000 visitors a year to its shallow, gin-clear waters and variety of sea life, from ballerina-like spotted eagle rays to lumbering Nassau groupers, an endangered species threatened by overfishing.

"I've seen good changes at Hol Chan," says longtime San Pedro snorkel guide Erlindo Alphonso ("Lil' Alphonso") Graniel, who wows clients by cradling the docile nurse sharks he attracts from dangled bait at the reserve's aptly named Shark Ray Alley. When the park doubled its user fee to $10 a person a few years ago, officials noted a sharp drop-off in cruise ship visits that had strained resources.

But severe coral bleaching in 1998, followed by a hit from Category 5 Hurricane Mitch the same year, ravaged a wide swath of Belize's reefs, including Hol Chan. Adds Paz: "You could have a million snorkelers touching coral, and they wouldn't do the damage of a single bleaching event. Belize has done a lot of good things, from training its guides to using mooring buoys instead of anchors, but global warming is a global issue."

A three-hour boat ride southeast of Ambergris Caye, the Belize reef system encompasses Turneffe Atoll, a 30-mile-long, 10-mile-wide maze of lagoon and more than 200 mangrove islands that ranks as one of the Caribbean's largest and most diverse marine ecosytems.

Apart from a smattering of seasonal fishing and lobster camps and regular visits from dive boats, civilization at Turneffe is limited to three low-key lodges and a research field station run by San Francisco-based Oceanic Society Expeditions, a non-profit outfit that leads natural history trips around the world.

Under the tutelage of scientists, small groups of curious travelers spend their days wielding clipboards and snorkels to learn about coral reef and mangrove ecology and such local denizens as bottlenose dolphins and crocodiles.

Evenings, they retreat to weathered wooden cabanas where entertainment consists of counting stars, scratching no-see-'em bites and listening to the lulling roar of waves breaking on the reef just a few yards from the beach.

One of the most popular excursions from Turneffe is Lighthouse Reef, another atoll 90 minutes to the east whose secluded islets and stretches of azure waters are home to a sizable bird rookery at Half Moon Caye Natural Monument and the country's most celebrated dive destination, the Blue Hole.

Made famous after a 1972 visit by Jacques Cousteau, the oval remnant of a collapsed limestone cave consists of sheer walls that plummet to more than 400 feet — though most divers are content to peer into the abyss from a forest of stalactites and stalagmites some 100 feet beneath the surface.

As visitors to Oceanic's field station learn early on, even these remote Belizean outposts aren't immune to the pressures affecting Ambergris Caye and other places that depend on the coral reef for their economic survival.

A stroll along Turneffe's powdery beaches leads past more than conch shells and skittering crabs: constant deposits of mostly plastic flotsam and jetsam, carried by currents from the far corners of the globe. And grad student Katheryn Patterson's lecture on the atoll's history includes a warning about its future: a wave of development that includes a proposed 4,800-acre planned community and golf course that could infringe on crucial fish and wildlife habitats.

Belizean Joseph Walker, whose long exposure to sun and sea make him seem older than his 63 years, spends at least eight months a year as a caretaker at a fishing camp overlooking Turneffe's lagoon. He says he has seen a decline in fish over the years, though he blames overzealous commercial fishermen more than climate change.

As for what's coming in on the next tide, "I just hope I live to be 120," says Walker. "I want to see whether things will work out."